During last month's Republican presidential debate in Las Vegas, frontrunner Donald Trump passed on an opportunity to assail his ascendant rival, Sen. Ted Cruz of Texas. Though Trump had dinged Cruz just two days before for acting like a scorched-earth "maniac" in the Senate, the billionaire showman decided by the time of the Dec. 15 debate that Cruz, in fact, had "a wonderful temperament."

But as Cruz's lead in the kickoff Iowa caucuses shows signs of durability and polls show him inching closer to or catching Trump in states like South Carolina and California, Trump is indicating that he may be finished playing nice with Cruz.

Though Trump has yet to wage the kind of concerted attack campaign against Cruz that he did to take down the "low-energy" Jeb Bush and the "pathological" Ben Carson, we've seen glimpses in recent weeks of the kind of fight Trump may wage against Cruz — and it looks awfully familiar.

Much as Trump lobbed nakedly nativist attacks against President Barack Obama during his 2011 flirtation with a presidential bid, peddling the conspiracy theory that Obama was born outside the United States, Trump has sought in recent weeks to depict the Canadian-Cuban-American Cruz as a vaguely foreign "other," one who may be neither culturally appropriate nor legally qualified to carry the GOP banner this fall.

Birtherism redux: The clearest indication that Trump would adopt this tack came Monday, when hetoldthe Washington Postthat Cruz's Canadian birth presented Republicans with a "very precarious" situation.

"Republicans are going to have to ask themselves the question, 'Do we want a candidate who could be tied up in court for two years?' That'd be a big problem," Trump said. "It'd be a very precarious one for Republicans because he'd be running and the courts may take a long time to make a decision. You don't want to be running and have that kind of thing over your head."

While Trump's questioning of Cruz's eligibility to serve as president hinted at a new rupture in the pair's relationship, it didn't mark the first time Trump has dabbled in Cruz birtherism. After Cruz launched his presidential bid in March, Trump called Cruz's Canadian birth "a hurdle that nobody else seems to have at this moment."

"He was born in Canada, if you know, and when we all studied our history lessons, you're supposed to be born in this country, so I just don't know how the courts would rule on it," Trump said, according to Salon. As Bloomberg Politics noted at the time, Trump also suggested in 2013 that Cruz was "perhaps not" eligible to serve.

The son of a Cuban father and a United States-born mother, Cruz is widely understood to meet the Constitution's requirements for "natural-born" citizenship, given his mother's nationality.

Not-so-thinly veiled xenophobia: There are obvious differences between Trump's Obama birtherism and his Cruz birtherism, as the Washington Post's Aaron Blake pointed out. The latter stems from the fact that Cruz was indeed born in a foreign country (albeit to an American parent), while the former is based on the unfounded conspiracy theory that the United States' first African-American president is a foreign-born usurper.

Still, it's hardly unreasonable to wonder whether there's an ethnic component to Trump's jabs at Cruz's lineage, given Cruz's Hispanic heritage and Trump's derision of Mexican immigrants as criminals and "rapists." And you don't have to look far to find examples of Trump pointing out — he's just saying it! — that Cruz is of Cuban descent.

The first instance came last month, during a campaign rally in Des Moines, Iowa, as Cruz increasingly evinced strength in the Hawkeye State thanks to his popularity with evangelical voters.

"I do like Ted Cruz, but not a lot of evangelicals come out of Cuba," Trump said, according to CNN.

It's a line Trump has repeated in some form several times now. Late last month, Trump all but pleaded with Iowans to keep Cruz's heritage in mind before they headed to the caucuses, urging, "Just remember this — you gotta remember, in all fairness, to the best of my knowledge, not too many evangelicals come out of Cuba, OK?" the Washington Times reported.

The Cruz campaign's response to such attacks has largely been muted, perhaps out of calculation that calling out Trump for xenophobia will harm Cruz's efforts to win over the largely white, working-class base of Trump voters.

"I'm a little confused by it; I'm not sure what it means," Cruz spokesman Rick Tyler responded to CNN after Trump's "just remember" plea. "Does it mean if you're a Cuban you can't be an evangelical or if you're an evangelical you can't be a Cuban? I can't make any sense of it."

Those who remember Trump's role as birther-in-chief may not have as much difficulty making sense of it.